The invention relates to a safety device which can prevent electronic equipments loaded on vehicles, for example, from malfunctioning in electrostatic environment of high level.
With development of the electronics industry, a quite large number of electronic devices are put into use everywhere in recent years. Such electronic devices include numbers of units for delivering large electric energy into an atmosphere in the form of signals to be transmitted, such as communication equipments, and units releasing relatively large noises into an atmosphere in the form of electric energy due to switching of large electric power. In an atmosphere, therefore, electric energy of relatively large level is present in the form of electromagnetic waves.
In these days, there are also employed numerous electronic devices handling a minute signal, which are susceptible to electric energy or noises in an atmosphere. Stated otherwise, electric energy in an atmosphere induces an electric signal (noise) on conductors, particularly electric wiring patterns, contained in such electronic device, as a result of which the device will malfunction when the noise level becomes equal to or higher than a level of the signal handled by the device.
In case of household equipments which may malfunction due to an influence of electromagnetic waves, for example, the above problem can be overcome for each equipment by arranging such as a wireless transmitter radiating electromagnetic waves in a position far away from the concerned equipment, because it is placed to be stationary. However, in case of electronic equipments which are loaded on mobile vehicles such as automobiles, the equipment may be subjected to the electric field of extremely high level dependent on a location of the automobile along its cruising route. Also, when a wireless apparatus is loaded on the automobile, it is practically impossible for the wireless apparatus to be placed far away from the concerned equipment which may suffer from disturbances. Japanese Automobile Electronics Research Committee has confirmed that an electronic equipment on automobiles is subjected to an environment possibly undergoing the electric field in order of 60 (V/m). It is very important for safety that such an electronic equipment on automobile is designed so as not to malfunction, particularly when it has a control function in relation to movement of the automobile. From this reason, an electronic equipment loaded on automobiles requires to be tested under an environment in which the electric field in order of the above-mentioned level is applied.
In the past, malfunctioning of electronic equipments in electrostatic environment of high level has been coped with as follows:
(a) To avoid electromagnetic waves from entering the equipment, an electronic circuit section in the equipment is entirely covered with a metal for electrostatic shielding; and
(b) A capacitor allowing needless components of high-frequency signal to pass therethrough is attached to each of all those parts in an electronic circuit which are especially susceptible to the electric field.
However, the method (a) cannot completely shield the connecting section of input and output lines used for connecting the concerned electronic circuit to another unit. For the method (b), such a capacitor cannot be attached to the parts handling a high-frequency signal. Moreover, the methods (a) and (b) are both disadvantageous in that the number of components is increased, design and manufacture of the equipment are complicated, and the cost is raised up correspondingly.